1. Field of the Invention
This invention relates to a light apparatus for use with an equipment for projecting uniform light in a particular direction, such as a liquid crystal projector, an OHP (overhead projector), or the like.
2. Description of Related Art
Various light apparatus are already known. An exemplary one of such conventional light apparatus is shown in FIG. 5. Referring to FIG. 5, the conventional light apparatus shown is constructed for use with an an overhead projector and includes a reflecting plate 1 formed in a spherical shape of an arcuate section and a discharge tube 2 in the form of a bulb such as a halogen lamp disposed at the center of the sphere of the reflecting plate 1. Thus, part of light emitted from the discharge tube 2 and advancing rearwardly toward the reflecting plate 1 is reflected forwardly by the reflecting mirror 1 so as to make the most of the light emitted from the discharge tube 2.
Another exemplary one of a conventional light apparatus is shown in FIG. 6. Referring to FIG. 6, the conventional light apparatus shown is employed for an equipment wherein parallel rays of light are required, such with as a liquid crystal projector, and includes an elongated discharge tube 3 such as a metal halide lamp and a reflecting plate 4 of a paraboloid of revolution disposed behind and around the discharge tube 3 such that part of output light of the discharge tube 3 (light emitted in a hatched area U in FIG. 6) is reflected by the reflecting plate 4 so as to produce parallel rays of light.
In such light apparatus which produce rays of light in a particular direction as described just above, part of the light emitted from the discharge tube 3 is reflected by the reflecting plate 4 so that light may be outputted in the particular direction, that is, in the rightward direction in FIG. 6. However, light emitted from the discharge tube 3 and advancing rearwardly toward the reflecting plate 4 partially leaks rearwardly through a center hole 4a of the reflecting plate 4 in which the discharge tube 3 is fitted. Consequently, light which is actually outputted in the particular direction of the light apparatus is part of the light emitted from the discharge tube 3. Accordingly, the utilization factor of emitted light is low.
Thus, in order to provide a solution to the problem described above, the inventor of the present invention has proposed such a light apparatus as shown in FIG. 7 in Japanese Patent Application No. 2-272153. Referring to FIG. 7, the light apparatus includes a reflecting plate 5 formed as a spheroid or ellipsoid of revolution. The reflecting plate 5 has a light discharging hole 6 formed at a portion thereof at an end of the major axis of the spheroid, and a discharge tube 7 is assembled to the reflecting plate 5 such that it is positioned at either one of two focuses F.sub.1 and F.sub.2 of the spheroid and extends perpendicularly to the major axis of the spheroid. With the light apparatus, most of the light generated from the discharge tube 7 is outputted in a particular direction, that is, in the rightward direction in FIG. 7, through the light discharging hole 6.
However, in order to manufacture the reflecting plate 5 of the light apparatus, a first reflecting plate 5a and a second reflecting plate 5b which are a pair of longitudinal halves of the reflecting plate 5 are formed separately from each other. Then the first and second reflecting plates 5a and 5b are coupled to each other at opening faces thereof along a plane perpendicular to the major axis of the spheroid at the center of the major axis in order to make the reflecting plate 5. Further, the second reflecting plate 5b in which the light discharging hole 6 is formed and on which a mounting portion for the discharge tube 7 is provided has a cutaway portion 8 of a substantially U-shaped section formed at an end portion thereof as shown in FIG. 8 at a following step. Then discharge tube 7 is inserted into and secured to the cutaway portion 8 of the second reflecting plate 5b, whereupon an end of the cutaway portion 8 is left as the light discharging hole 6. Accordingly, at the location of the reflecting plate 5 where the cutaway portion 8 is formed, light cannot be reflected, and accordingly, light emitted from the discharge tube 7 and advancing to the cutaway portion 8 is directly discharged outwardly and cannot be outputted in the particular direction. Consequently, the utilization factor becomes deteriorated. Further, since the manufacturing method not only requires such a following step as described above but also involves machining of the reflecting plate 5, which is comparatively small in thickness, the manufacture of the light apparatus is complicated and is low in mass productivity.
Since it is necessary to make the occupied area of the cutaway portion 8 as small as possible in order to assure a high utilization factor of light, the discharge tube 7 is formed as a one layer tube and has its light emitting portion exposed outwardly. Consequently, if a finger inadvertently touches a surface of the light emitting portion of the discharge tube 7, the transparency of the discharge tube where the finger touches may be deteriorated into a non-transparent condition to thereby decrease the output of light. Accordingly, care must be taken for handling of the discharge tube 7. Further, when the discharge tube 7 is to be exchanged, for example, due to its failure, such exchanging operation must be performed carefully, which is cumbersome.